In U.S. Pat. No. 4,484,179 issued to L. R. Kasday, there is disclosed, inter alia, an optically-based touch-sensitive screen for determining the position of a finger touch on the surface of a CRT screen. This arrangement includes a transparent parallel surface device for overlaying the CRT screen and through which light from the CRT may pass. When a point on the device is touched, light from the CRT raster beam becomes trapped within the device by total internal reflection. This trapped light travels to the edge of the device and is detected by photodetectors mounted along the edge of the device. Since the total internal reflection occurs at the time the CRT raster beam passes under the finger, the raster beam timing can be used to determine the position of the touch.
Disadvantageously, imperfections in the material used in the optically-based touch screen, as well as dust, fingerprints, smudges, etc., on the touch screen itself, all good combine to cause some total internal reflection of the CRT raster beam when the screen is not touched. Consequently, a problem arises of distinguishing trapped light signals that occur when a displayed image is touched from trapped light signals that occur when the image is not touched.
This problem is compounded if an image is displayed at a bright spot on the CRT, since the luminance of the bright spot causes the level of trapped light to increase. This increase can thus lead to a false indication that a displayed image has been touched when in fact it has not been touched by the user.
One possible approach to the problem of determining when a screen is touched would be to use some form of adaptive thresholding to detect a sought-after signal. In such an arrangement, a voltage threshold would be adapted to the level of the background, or non-sought-after, signals "on the fly" and the level of a received signal would be compared with the level of the threshold. Only if the received signal exceeds the adaptive threshold would it be identified as being the sought-after signal. Typical optically-based touch screen arrangements display particular regions on the screen, referred to as "targets", for touch by the user and a trapped light signal that comes from a target that is touched needs to be distinguished from one that comes from a target that is not touched. In such arrangements, adapting the aforementioned threshold "on the fly" as previously described may give rise to a number of problems.
For example, a target might be displayed at a so-called CRT bright spot, possibly resulting in an untouched target signal level that exceeds the threshold and therefore resulting in an erroneous indication that the target had been touched.
A problem may also arise in the above-described scheme when a new ensemble of displayed targets is displayed. The threshold value that obtains immediately upon the target change will, for some time, be based, at least in part, on the old targets until the threshold has had a chance to adapt to the level of the signals from the new untouched targets. The intensities of the new untouched targets may, however, be different from those of the old because the luminance of the CRT phosphor may vary from one location of the CRT screen to another. Again, the level of an untouched target signal could exceed the threshold and be misinterpreted as coming from a touched target.